Electrolysis of water, that is, splitting water into oxygen and hydrogen gases, is a very important process not only for the production of oxygen and/or hydrogen gases, but for energy storage. Energy is consumed in splitting water into hydrogen and oxygen gases and, when hydrogen and oxygen gases are re-combined to form water, energy is released.
In order to store energy via electrolysis, materials are required which efficiently mediate the bond rearranging “water splitting” reaction to O2 and H2. The standard reduction potentials for the O2/H2O and H2O/H2 half-reactions are given by Equation 1 and Equation 2.
                                                                                          O                  2                                +                                  4                  ⁢                                      H                    +                                                  +                                  4                  ⁢                                      e                    -                                                              ↔                                                H                  2                                ⁢                O                                                                                        E                0                            =                                                +                  1.23                                -                                  0.059                  ⁢                                                                          ⁢                                      (                    pH                    )                                    ⁢                  V                                                                                        (        1        )                                                                                    2                ⁢                                  H                  2                                            ↔                                                4                  ⁢                                      H                    +                                                  +                                  4                  ⁢                                      e                    -                                                                                                                          E                0                            =                              0.00                -                                  0.059                  ⁢                                                                          ⁢                                      (                    pH                    )                                    ⁢                  V                                                                                        (        2        )                                                      2            ⁢                          H              2                                +                      O            2                          ↔                  2          ⁢                      H            2                    ⁢          O                                    (        3        )            For a material to be efficient for this conversion, the material should operate at a potential close to the thermodynamic potential of each half-reaction, which are defined by half-cell potentials, Eo in Equations 1 and 2. Voltage in addition to Eo that is required to attain a given activity, referred to as overpotential, limits the conversion efficiency and considerable effort has been expended by many researchers in efforts to reduce overpotential in these reactions.
Hydrogen exhibits many advantages as a carbon-free energy storage medium: it has the highest energy density by mass of all chemical fuels, and it may be readily formed in a sustainable fashion via the electrolysis of water powered by renewable energy. However, despite the conceptually simple generation of hydrogen by water electrolysis, the energy efficiency and capital cost associated with water electrolysis have limited commercial applications, thus many attempts have focused on the improvement of the electrolytic generation of hydrogen. In particular, many electrodes that are used for hydrogen production may be unstable and/or inefficient for forming hydrogen gas from water comprising additives and/or impurities.
Accordingly, improved methods, systems, and electrodes are needed.